In commonly assigned U.S. Pat. No. 3,883,046, there is disclosed an improved positive expulsion tank featuring a bladder having an as-fabricated tubular or cylindrical configuration. The bladder is end mounted within the tank on a centrally disposed assembly consisting of a pair of facing, axially spaced conically shaped members, which are apertured to permit the introduction and withdrawal of the product from within the confines of the bladder, and a product transport tube, which serves to connect the apexes of the members and may, if desired, also be apertured. The ends of the bladder are secured peripherally to the base of each of the conically shaped members, such that the bladder is essentially relaxed when it assumes its as-fabricated cylindrical configuration in which it is disposed concentrically outwardly of the transport tube. In this configuration of the bladder, it cooperates with the transport tube to define a partial tank-full cavity. When filling the tank, the liquid product introduced into the bladder will initially fill this cavity and then serve to expand the bladder outwardly until it assumes a highly tensioned, tank-full configuration.
The tank is emptied by applying an expelling fluid, such as a gas, to the tank exterior of the bladder. During resultant discharge of stored product through the transport tube, the bladder first returns to its essentially relaxed, as-fabricated cylindrical configuration and is thereafter again immediately tensioned as it is forced by the expelling fluid to assume a tank-empty configuration wherein it lies in conforming engagement with the surface of the transport tube. Thus, for each product discharge cycle, the bladder is exposed to two tension modes, during which the elastic properties of the bladder prevents the formation of failure producing uncontrolled buckles and folds. Further, the design as such that the bladder is not subject to abrading contract with any constraint or guide throughout substantially the whole of its range of deformation.
Since the filing of the patent application, which has now issued as above mentioned U.S. Pat. No. 3,883,046, the behavior of various bladder materials, while exposed to diverse propellants or fuels under a wide range of temperature storage conditions, have been the subject of intensive study. As a result, it has been determined that many product compatible bladder materials, which are capable of undergoing resilient deformation under normal and even relatively high temperature product storage conditions, are subject to becoming "set" or suffering a temporary loss of elastic memory or recovery ability after being maintained under relatively low temperature product storage conditions, as for instance about minus 65.degree. F. When a product filled bladder becomes set in this manner, it has been found subject to permanent damage or complete failure, if forced to undergo a change in shape, such as would occur when the stored product experiences a high amplitude slosh mode resulting from sharp movements or accelerations of the expulsion tank during handling or transport thereof. Although the bladder materials under investigation have been found to be capable of eventually recovering their elastic memories after being returned to room temperature storage conditions for periods ranging from ten minutes to four hours or more, their low temperature-high stress failure characteristics have been found to place a practical limit on the versatility of positive expulsion tanks of a general type disclosed in the above-mentioned patent.